This invention relates to a method of producing a partly colored and at least partly curved glass sheet, in which method a colored coating layer is formed simultaneously with press-shaping of glass sheet. This method is suitable to the production of window glasses for vehicles.
For vehicle windows and particularly for rear and side windows of automobiles, recently there is an increasing trend of employing a partly colored window glass, which is a curved glass sheet having a colored and opaque or translucent coating layer formed for ornamental or aesthetic purposes on a surface of the glass sheet to become the inboard side of the window glass in a marginal region along the entire periphery.
Usually such a colored coating layer is formed at the stage of press-shaping a flat glass sheet of a definite profile into a desirably curved sheet, by using a colored paste prepared by kneading a mixture of an inorganic pigment, a glass frit of a relatively low melting temperature and a vehicle such as an oil. As a minor modification, the inorganic pigment may be incorporated into the glass frit by adding the pigment to a powdery composition for the preparation of the frit. First, the paste is applied onto the flat glass sheet by, for example, a screen-printing technique to cover the intended marginal region of the glass surface with a thin layer of this paste. After drying of the colored paste layer, the glass sheet is heated to a temperature sufficient for softening of the glass sheet and melting of the glass frit contained in the colored paste. Then the heated glass sheet is subjected to a usual press-bending operation wherein a male shaping tool is pressed against the paste-applied surface of the glass sheet. The fused glass frit serves as a binder for the inorganic pigment particles and strongly adheres to the glass sheet surface. When cooled, the marginal region of the concave side of the curved glass sheet is coated closely with a colored and film-like glass layer.
However, practical applications of this method revealed a problem that often difficulty arises in parting of the shaping tool from the press-shaped glass sheet because of adhesion of the tool to the glass sheet. Since an unwanted pulling force is exerted on the shaped glass sheet in such a case, the shaped glass sheet tends to undergo unintended deformation and becomes unsatisfactory in the precision of its curved shape. The reason for adhesion of the shaping tool to the shaped glass sheet is a fact that during the press-shaping operation the fused glass frit as a binder component of the coloring agent adheres not only to the heated glass sheet surface but also to the shaping tool. In other words, the fused layer of the coloring agent works as an adhesive layer between the shaping tool and the glass sheet.